1. Field of the Invention
The present invention relates to a stabilizer for a synchronous electric power generator system, and in particular to a power system stabilizer providing both power system damping functionality and excitation limiter functionality from a single unit.
2. Background Information
Synchronous electric power generators are well-known in the art and are used to produce alternating current output voltages, such as single phase or three phase voltage outputs. A typical synchronous electric power generator has a stator and a rotor having a field winding excited by a field voltage provided by an exciter. Current in the rotor creates a rotating magnetic field which induces current in the stator to produce an output voltage at the terminals of the stator. The amount of field voltage provided by the exciter controls the generator field magnetic strength. As the main generator field strength is controlled, so is the generator output voltage induced in the generator stator. In the typical case, the exciter is under the control of an automatic voltage regulator which is connected to the output voltage terminals of the stator. The automatic voltage regulator controls the voltage applied to the exciter which adjusts the generator output voltage by controlling the strength of the main generator field. Many different techniques are used to sample or sense the output voltage and provide feedback to the automatic voltage regulator, one example of which includes detecting the peak of every other half cycle of the output voltage, comparing it with a desired reference value, and then adjusting the exciter voltage up or down as required to correct the generator output voltage.
In modern generator systems, the voltage regulator/exciter system for the generator (typically provided as a single electronic component) also provides various control functions for further controlling the level of excitation and thus the generator field strength. Such functions are commonly referred to as excitation limiter functions and include, without limitation, a minimum excitation limiter (MEL) function, an under excitation limiter (UEL) function, a volts per hertz (VHZ) function, a generator current limiter (GCL) function, and an over excitation limiter (OEL) function.
In addition, in large power generator installations, the utilization of quickly responding electronic equipment for control of the generator field excitation has lead to undesired stimulation of mechanical and electrical resonances resulting in power system oscillations. This problem has been found to be particularly severe when multiple generators are electrically coupled together or when long transmission lines are required to distribute the generator power. The problem is generally overcome by the inclusion of a compensating control element called a power system stabilizer (PSS) in the feedback loop which regulates the generator output. A PSS is an auxiliary control device used with generator excitation systems to provide an additional signal to the voltage regulator to damp the turbine generator to power system electromechanical oscillations. In most systems, the output of the PSS is fed to a summing point within the voltage regulator and serves to modulate the voltage regulator output and thus control the generator excitation such that a damping torque is applied to the generator rotor to control generator oscillations. Operation of a PSS is well understood in the industry and the various implementation means have been well documented. Typically, a PSS takes generator voltage and current (received from voltage and current transducers, respectively) as inputs to generate the auxiliary signal provided to the voltage regulator. In addition, other possible inputs to a PSS include the deviation of the rotation speed of the generator (from a speed transducer) and the deviation of the system frequency.
Most older generator systems (twenty to twenty years old and older typically utilize a voltage regulator/exciter system that is not equipped to provide the excitation limiter functions described above. In addition, such systems do not normally employ a PSS to damp generator oscillations as described above. Thus, an operator of such an older generator system that desires to add excitation limiter and PSS functionality would need to both replace the existing voltage regulator/exciter system with a voltage regulator/exciter system that is equipped to provide the excitation limiter functions and purchase a PSS, even if the existing voltage regulator/exciter system is functioning properly. There is thus a need for a system that can provide both excitation limiter functions and PSS functions in a single unit that may be used, for example, in older generator systems and thereby enable continued use of a properly functioning existing voltage regulator/exciter system.